Semiconductor memory devices may be broadly categorized as either volatile or nonvolatile memory devices. The volatile memory devices include static random access memory (SRAM) and dynamic random access memory (DRAM). In static random access memory, data is stored by setting the logical state of a bistable flip-flop. In dynamic random access memory, data is stored by charging a capacitor. Volatile semiconductor memory devices can only store data while power is applied to the device, and the data is lost whenever the power is interrupted.
Nonvolatile semiconductor memory devices such as mask read only memory (MROM), programmable read only memory (PROM), erasable read only memory (EPROM), electrically erasable read only memory (EEPROM), and the like, can store data even when power is not applied to the device. Some nonvolatile memory devices store data permanently, while other may be reprogrammed. Nonvolatile semiconductor memory devices are widely used to store programs and micro code in various technical areas such as computer sciences, avionics, communications, consumer electronics, etc.
Volatile and nonvolatile memories may be combined and applied in one chip for use in systems requiring fast reprogrammable nonvolatile memory. An example of such a memory is a nonvolatile RAM (nvRAM). In addition, specific memory structures including additional logic circuits have been developed to optimize memory performance in application oriented processes.
MROM, PROM, and EPROM generally cannot be reprogrammed by a typical user due to the requirements of the erase and write operations for these devices. However, EEPROM can easily be electrically erased and rewritten. Hence, EEPROM is widely used for data storage in systems that require continuous updating, and in supplementary memory devices. EEPROM (hereinafter, referred to as “flash memory”) has a higher integration density compared to conventional EPROM and may be used as a high capacity supplementary memory device.
Flash memory typically requires voltages that are higher than the power supply voltage to carry out erase, program and read operations. These high voltages are typically generated by pumps on the flash memory. Thus, a single memory device mav include a high voltage pump, a program voltage pump, an erase voltage pump, a read voltage pump, etc. These pumps occupy large amounts of space on the devices, thereby reducing the overall integration density of the flash memories.